The "patient-centered medical home" (PCMH) has been proposed as a solution to the numerous challenges facing the US primary care system. This new model is being aggressively adopted by many physician practices and organizations to improve quality, efficiency, and the experiences of both patients and their health care providers. Originally promoted by medical professional organizations, it has been widely endorsed by health care systems, provider groups, public and private payers, employers, and others. While the research base on the importance of primary care to has been firmly established, there remains little peer-reviewed empirical evidence on outcomes from a PCMH, across different types of practices, the magnitude or timing of the effects, or the optimal way to support or guide practices through change This study proposes a rigorous, mixed method examination of a large PCMH transformation that involves spreading an ambitious set of changes across a health care system, Group Health Cooperative, with a large number of patients and a diverse array of practices. The study builds on evaluation work that is already underway and takes advantage of a unique opportunity to study the real-world spread of a PCMH across an entire health care system. The specific aims are to: (1) assess the PCMH effects on quality and efficiency, (2) assess the PCMH effects on patient and staff experiences, and (3) document to PCMH implementation methods and accommodation of contextual factors. To achieve our aims, we will: (1) collect administrative data on utilization, cost, and quality to examine the relationship between successful implementation of the PCMH and changes in key process and outcomes measures, (2) conduct focus groups with patients (with both high and low illness levels) regarding their experiences with the PCMH, (3) collect survey data from primary care health care staff across Group Health's primary care clinics, (4) conduct qualitative interviews and ethnographic observations with Group Health's primary care leaders and managers to document the PCMH implementation from a system perspective, (5) conduct qualitative interviews at and site visits to 9 different clinics to document the clinics'PCMH experiences and the effect of the transformation on the clinical staff. Results from this study will provide important findings regarding potential impact of transforming primary care through the spread of a PCMH model on health care as well as lesson regarding methods for implementation. PUBLIC HEALTH RELEVANCE: Despite rapidly being adopted across the US, little is known about the effects of the patient-centered medical home (PCMH) across different types of medical practices, nor about how to effectively conduct the practice transformation. This study takes advantage of a unique natural experiment across one large health care system to rigorously study the effects and process of PCMH transformation on quality of patient care, patient and clinical staff experiences and cost and quality. The evaluation is relevant to policy makers, health care systems, medical groups, and government agencies considering or working on similar implementations.